1. Field of the Application
The present invention relates to the field of broadband cellular communication systems, and in particular to techniques for facilitating efficient handoff and data throughput, such as using selectively enabled soft handoff, performing Layer 2 bearer functions at the base station and using the mobile device to coordinate soft handoff and interference avoidance without the need for a centralized coordination function.
2. Background of the Disclosure
Soft handoff (“SHO”) is a macro-diversity scheme in which the same information is sent to the BTSs via multiple base stations, as opposed to selecting the best station for transmission time to time (as in fast cell selection). Soft handoff may be described generally as transmitting the same information via multiple base stations, as opposed to selecting the best station for transmission from time to time (an example of the latter is fast cell selection). The multiple base stations are selected by the mobile device on the basis of specified signal strength criteria. If selected, a base station becomes a member of the mobile device's active set.
Normally, when a mobile device enters SHO, all active services at the mobile device are supported by SHO. The basis of conventional SHO is the establishment of virtual circuits at the multiple active set base station transceiver systems (“BTSs”). Base station transceiver systems are generally known in the art. Of note, SHO is not defined for the “fat-pipe” packet based versions of current systems (e.g., downlinks of 1×EV-DO/DV and Universal Mobile Telecommunications System High Speed Data Packet Access (“HSDPA”) systems). 1×EV-DO/DV is a CDMA2000 standard.
Techniques for supporting soft handoff in a general fashion are known. For example, the conventional procedure for triggering soft handoff involves the maintenance of an “active set” of BTSs at each mobile device. A BTS becomes a candidate set member when the pilot signal strength of that BTS exceeds a predetermined value, referred to as a T_ADD value. A BTS is no longer a member of an active set when its pilot signal strength falls below another predetermined value, referred to as T_DROP value, and remains below the T_DROP value for a period defined by the handoff drop timer. (see, for example, Sec. 2.6.6.2.3, TIA-2000.5-D). The mobile resets and disables the handoff drop timer if the strength of the corresponding pilot exceeds T_DROPs. If the timer expires, the BTS is removed from the active set and the mobile device is no longer in SHO with that specific BTS. For adding new BTSs into the active set, the mobile device reports that a Candidate Set pilot is stronger than an Active Set pilot only if the difference between their respective strengths is at least a predetermined value, referred to for example by T_COMP.times.0.5 dB. Thus BTSs are added and removed from the active set on the basis of a set of triggers based on pilot strength.
Once BTSs enter an active set, SHO is initiated. The current trigger for SHO as per the Telecommunication Industries Association CDMA 2000 standard, TIA-2000.5-D, Upper Layer (Layer 3) Signaling Standard for CDMA2000 Spread Spectrum Systems, is as follows:                “When the Active Set contains more than one pilot, the mobile station should provide diversity combining of the associated Forward Traffic Channels” (p-2-586, TIA-2000.5-D).        
FIGS. 1 & 2 are derived from TIA-2000.5-D, relating to the eligibility of BTSs to enter or leave active sets are discussed herein to provide a more specific example of inclusion/deletion from an active set. TIA-2000.5-D, in its entirety, is herein incorporated by reference. Referring first to FIG. 1 which shows a typical pilot signal lifecycle for a corresponding BTS, at time (1), the pilot signal strength is greater than T_ADD and the mobile device sends a Pilot Strength Measurement Message and transfers the pilot to the Candidate Set of BTSs. At time (2), the base station sends an Extended Handoff Direction Message, a General Handoff Direction Message or a Universal Handoff Direction message to the mobile device. At time (3), the mobile device transfers the BTS corresponding to the pilot to the Active Set and sends a Handoff Completion message. At time (4), the pilot strength drops below T_DROP and the mobile device starts it handoff drop timer. The handoff drop timer expires at time (5) and the mobile device sends a Pilot Strength Measurement Message to the BTS. At time (6), the base station sends an Extended Handoff Direction Message, a General Handoff Direction Message or a Universal Handoff Direction message to the mobile device. At time (7), the mobile device moves the BTS corresponding to the pilot signal from the Active Set to the Neighbor Set and sends a Handoff Completion Message to the BTS.
FIG. 2 shows typical pilot signal lifecycles for pilot signals P1 and P2 corresponding to multiple BTSs. At time (1), pilot P2 strength exceeds T_ADD and the mobile device transfers the BTS corresponding to the pilot signal to the Candidate Set. At time (2), the pilot signal P2 strength exceeds ((SOFT_SLOPE/8)*10*log.sub.10 (PS1)+ADD_INTERCEPT/2) and the mobile device sends a Pilot Strength Measurement Message to the BTS. At time (3), the base station corresponding to pilot signal P2 sends an Extended Handoff Direction Message, a General Handoff Direction Message or a Universal Handoff Direction message to the mobile device, the mobile device transfers the BTS corresponding to pilot signal P2 to the active set and sends a Handoff Completion Message to the BTS. At time (4) the pilot signal P1 strength drops below ((SOFT_SLOPE/8)*10*log.sub.10 (PS2)+DROP_INTERCEPT/2), and the mobile station starts the handoff drop timer. The handoff drop timer expires at time (5) and the mobile device sends a Pilot Strength Measurement Message to the BTS corresponding to pilot signal P1. At time (6), the BTS corresponding to pilot signal P1 sends a sends an Extended Handoff Direction Message, a General Handoff Direction Message or a Universal Handoff Direction message to the mobile device, the mobile device transfers the BTS corresponding to pilot signal P1 to the Candidate Set and sends a Handoff Completion Message to that BTS. At time (7), the pilot signal P1 strength drops below T_DROP and the mobile device starts the handoff drop timer. The handoff drop times expires at time (8) and the mobile device moves the BTS corresponding to pilot signal P1 from the Candidate Set to the Neighbor Set. Additional details regarding known soft handoff techniques can be found in the CDMA 2000 standard TIA-2000.5-D.
While SHO can be very beneficial to improve a mobile device's carrier-to-interference ratio (“C/I”) condition, the network and processing resources demanded through the implementation of SHO are significant. On the downlink (from BTS to mobile device), two or more BTSs are required to transmit the same information to a user which consumes resources in the BTSs and increases the interference seen by all users within the coverage area of those BTSs. On the uplink (mobile device to BTS), while code division multiple access (“CDMA”) systems only require additional processing and no specific scheduling of spectral resource, Orthogonal Frequency Division Multiple Access/Multiple Input Multiple Output (“OFDMA/MIMO”) systems operating with N=1 reuse will require explicit scheduling of uplink spectrum resources, as is done for the downlink.
In current designs, SHO is triggered on the basis of multiple BTSs in a mobile device's active set. Currently there is no association of SHO trigger with the type of service, e.g., real-time services require a consistent minimum C/I condition to minimize delays in transmission, while “best effort” services do not. This is the case because “best effort” services can afford multiple retransmissions and wait for the mobile device to enter an “up fade”. In an “up fade”, the C/I is higher than the average within the system, thereby increasing the overall channel capacity. The increased channel capacity provides support for “best effort” services.
Because SHO is demanding on resources, it is best utilized only on an as needed basis. Such an as needed basis is determined by the specific conditions that need to be met to support a given service. It is desirable to have a way to dynamically enter a subset of specific active services in a mobile device into SHO based on the current C/I conditions when the primary BTS is not able to sustain the service if other means to support the service (such as fast cell selection) have been exhausted.
A BTS may modify all handoff related parameters through the Extended Handoff Direction/completion message. There is only one set of parameters available for each mobile device and this set of parameters applies to all the services that the mobile device is actively supporting. There is no known system or method for specifically selecting SHO for certain services or quality of service (“QoS”).
It is desirable to have a system and method in which the SHO decision in packet based systems is made on the basis of both current radio link conditions and the requirements of the service(s) or associated data flows that the mobile device is actively supporting at the time of the decision. For example, if a mobile device is simultaneously engaged in a multimedia call with text, voice and video, it may be desirable that the SHO be triggered for the specific data flows of the voice and/or video services only. Additionally, SHO may be triggered only when other features, such as power stepping, adaptive modulation, fast cell selection macro-diversity, are inadequate to maintain the service quality.
In addition, while SHO can be very beneficial to improve a mobile C/I condition, there is a need for some form of centralized control to enable simultaneous transmissions from multiple BTSs using the same resource, i.e., radio channel, frequency and code. This requires co-ordination of the resources among participating BTSs. Typically, the coordination is done in a centralized manner using a network entity such as the radio network controller (“RNC”), which co-ordinates the functions of the BTSs.
There is no existing system or method for enabling base station transceiver resource co-ordination without the aid of a centralized network entity such as a RNC. Multi-base station resource co-ordination is usually done by a central entity such as a RNC. As such some Layer 2 resource functions, such as scheduling reservations for SHO, need to be made at a central common entity in the network.
The wireless communication industry is moving toward distributed architectures where the wireless network edge nodes do not require central control functions and are able to cover the intended coverage areas adequately and uniformly. However, because interference among neighboring sectors is very high (especially in reuse one systems), methods of interference avoidance or coordinated transmission techniques such as SHO are required to provide adequate coverage. Such techniques require centralized base station transceiver resource co-ordination and, therefore, cannot be implemented under a distribute architectures. One of the major challenges faced by the industry in moving toward distributed architecture is to improve coverage without using such centralized control functions.
There have been proposals which suggest moving all radio Layer 2 media access control/radio link protocol (“MAC/RLP”) functions to the BTS instead of carrying them out at a central entity. However, even in those proposals, simultaneous resource allocations needed in multiple base stations are still required to be carried out by a central entity. As such, these proposals still require a relatively complex access architecture, i.e., the protocols that are used to co-ordinate multiple BTS transmissions. In summary, with known SHO implementations or proposals, disadvantageously (1) either the RNC does all the scheduling, (2) the BTSs must send periodic updates to the RNC, or (3) slots must be reserved/tentatively reserved a priori by the RNC for SHO use. It is therefore desirable to simplify the access network architecture by allowing resource reservation by the mobile devices engaged in SHO instead of conducting such resource reservation via a central entity such as an RNC.
In some cellular networks such as those which support packet data traffic, there is no central entity to perform such coordination. This results in a cellular network design which therefore precludes the use of soft handoff or any other similar scheme requiring multi-base resource coordination to improve coverage. It is therefore desirable to have a system and method to provide a means to enable such flat networks to support soft handoff with the assistance of the mobile device with respect to the co-ordination of resources at multiple BTSs.
A detrimental characteristic of broadband cellular communication systems is that they suffer from uneven coverage. To improve coverage performance, cellular communication systems may employ macro-diversity such as the above-described soft handoff. The processing of soft handoff information may happen based on internet protocol (“IP”) packet selection diversity, radio link protocol (“RLP”) packet data unit (“PDU”) selection combining (1×EV-DV reverse link), or physical layer combining (1XRTT forward link and reverse link). 1×EV-DV and 1XRTT are CDMA 2000 standards. Soft handoff is naturally controlled by a central entity such as the RNC.
Current downlink (from base station to mobile device) soft handoff techniques used in power controlled CDMA systems e.g., 1XRTT, involve the multicasting of RLP PDUs from the RNC to BTSs, followed by simultaneous transmissions from multiple BTSs. The scheme is designed primarily for CDMA systems which are based on the RLP at the centralized RNC. It should be noted that soft handoff is a requisite feature for CDMA systems to address the resultant interference arising from the neighboring cell (using the principle of converting the interference to a wanted signal) and the near-far problem. The near-far problem is well known by those of ordinary skill in the art and is not explained herein.
Current uplink soft handoff techniques (from mobile device to base station), e.g., 1×EV-DO, include the simultaneous reception of information sent by a mobile device by multiple BTSs and forwarding of the RLP frames to the RNC where combining in the form of soft combining or selection combining occurs. For CDMA systems there is no requirement for explicit scheduling of resources on the uplink because the mobile device's unique pseudorandom noise (“PN”) code can be detected by any BTS. However, a receiver needs to be dedicated at each of the BTSs in the active set to demodulate the mobile device's transmissions.
Future networks may likely be based on a more distributed architecture, where the Layer 2 processing media access control/radio link protocol (“MAC/RLP”) is done exclusively at the BTS (e.g., Flarion). Furthermore, these networks may be operated with an OFDMA/MIMO air interface. The subject invention provides a solution to support soft handoff for OFDMA/MIMO rate controlled systems in which the Layer 2 (MAC/RLP) functions, i.e., segmentation/concatenation of packets, reassembly of packets, retransmission, reside exclusively at the BT. Advantageously, the present invention addresses both downlink (“DL”) and uplink (“UL”) soft handoff.
It is desirable to have a system which addresses the coordination of transmissions from/to different BTSs to/from the mobile device, so that multiple BTSs transmit/receive the same data to/from a given mobile device to obtain macro-diversity gain in a manner which does not require coordinated Layer 2 processing at a central location such as RNC. This design criterion impacts several aspects of the design such as the segmentation of data packets, the buffering of data packets, the resource reservation for given service, the simultaneous scheduling of a mobile device's packet in all the BTSs in the active set (this is applicable to both DL and UL transmissions), and the hybrid automatic repeat request (“HARQ”) or Layer 2 retransmissions for services in soft handoff.
Current DL soft handoff solutions for CDMA apply only to power controlled systems. However, there is a need to provide soft handoff solutions for other types of systems. There is also a need for centralized RLP processing and buffer management. However, current UL soft handoff solutions rely on a central entity such as a RNC to provide the coordination by forming the RLP packets such that sequence numbers for these packets are maintained at the centralized location. All negative acknowledgements (“NAKs”) or acknowledgements (“ACKs”) for the transmissions then terminate at the RNC. This method introduces undesirable complexity in the network architecture, and cannot be implemented in a distributed Layer 2 architecture.
There is currently no solution to enable soft handoff in a distributed architecture where the RLP processing is done at the BTS. It would be desirable to have a system and method which provides soft handoff in a distributed architecture in which the RLP processing is done at the BTS.